Add --target flag to the chrome_tests.py scripts.

Currently, --build-dir includes the target (Debug or Release) for these scripts,
but in all other scripts --build-dir just points to out / build / xcodebuild.
Make the valgrind scripts consistent with that.

Enables https://codereview.chromium.org/26184003 .

No intended functionality change.

BUG=294387
NOTRY=true

Review URL: https://codereview.chromium.org/26190002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227309 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227309